Composite material and method for producing same

ABSTRACT

A method for producing a glass or glass ceramic composite material is provided. The method includes: providing a glass or glass ceramic substrate with a coefficient of thermal expansion of α≦5*10 −6  K −1 ; applying a sol-gel layer onto the substrate; adding particles to the sol-gel layer; patterning the sol-gel layer; thermally curing the sol-gel layer; and applying at least one further layer using a deposition process.

FIELD OF THE INVENTION

The invention relates to a composite material, in particular a composite material comprising a glass or glass-ceramic substrate and a coating. In particular the substrate is at least partially transparent. The invention furthermore relates to a method for producing a composite material, in particular based on a glass or glass-ceramic substrate. The invention moreover relates to glass-ceramic articles having a decorative appearance, especially glass-ceramic cooktops, oven windows, architectural glazing, glass applications in consumer electronics, etc.

BACKGROUND OF THE INVENTION

Decorative coatings, especially at the bottom face of transparent glass-ceramics such as cooktops, are known from practice.

For example printed inks or lacquers such as silicone paints may be used. When using metal particles in the lacquer system the coating may have a metallic appearance. For this purpose, usually, a layer with a thickness of several microns has to be applied, whereby the coated area no longer exhibits a transparent appearance through which for example a backlight or a display would be visible.

Furthermore, when using metallic-looking lacquers the creative freedom is usually limited, and in many applications it is not necessarily desired for the metallic appearance to exactly correspond to that of a smooth metallic surface. Also, a combination of such lacquer systems with patterns in the lower micron range is often difficult, because the metallic pigments are often larger and do not permit to create a homogeneous color image. In particular, it would be desirable to achieve a color adaptation with other surfaces or kitchen appliances, such as (induction) cooktops.

In the kitchen area, for example, surfaces are commonly used that have an appearance of brushed stainless steel. For this purpose foils may be used, for example. However, such foils often fail to provide a high-quality appearance, and they are prone to delamination and discoloration, in particular when subjected to temperatures of more than 50° C.

Alternatively, stainless steel sheets may be used. However, these are relatively expensive, and a seamless transition between a glass-ceramic substrate and a stainless steel surface is usually not possible. Furthermore, moisture and dirt may penetrate between the layers thereby considerably deteriorating the appearance thereof.

Known decorative layers often fail to provide the necessary scratch resistance required to be used as an outside coating. When using a metallic-looking coating on the bottom face of a glass-ceramic article, in many cases a yellowish or reddish color cast impression is produced since the glass-ceramic article itself often has a color cast.

In addition, previous metal layers, due to their conductivity, often have the drawback of being not suitable for use in capacitive or similarly working display and/or touch display applications.

OBJECT OF THE INVENTION

An object of the invention therefore is to provide a simple and inexpensive method for applying decorative coatings with a high level of creative freedom, wherein such decorative coatings should in particularly exhibit a high temperature resistance.

Another object of the invention is to provide decorative layers with good adhesion to the substrate and a high level of scratch resistance. Also, the applied layers should not impair the strength of the substrate.

In addition, one embodiment of the invention is based on the object to provide layers with display and/or touch display capability.

SUMMARY OF THE INVENTION

The object of the invention is already achieved by a method for producing a composite material having at least two layers, and by a composite material according to any of the independent claims.

Preferred embodiments and refinements of the invention are set forth in the respective dependent claims.

The invention relates to a method for producing a composite material having at least two layers.

In particular, a glass or glass-ceramic composite material is used which resists to thermal shocks and has a coefficient of thermal expansion (CTE) of not more than 5*10⁻⁶ K⁻¹, preferably not more than 4*10⁻⁶ K⁻¹ (at 20° C.), and which is at least partially transparent.

This may be a material exhibiting high transmittance in the visible range as well as a nearly opaque material with only low transmittance.

Instead of a glass or glass-ceramic substrate, it is envisaged according to one embodiment of the invention to use a plastic substrate, in particular a transparent plastic substrate.

According to the invention, a patterned sol-gel layer, i.e. an at least partially inorganic layer, is applied using a liquid phase process.

Then, a further layer is deposited onto the patterned layer by a PVD or CVD deposition process, in particular by a PVD process.

The inventors have found that by combining a pattern-defining sol-gel layer and a deposited, for example color-defining layer a virtually unlimited creational freedom may be provided.

In particular, the deposited, further layer may be a colored layer or a layer having a metallic appearance, in particular a metal layer.

Thus, preferably, by virtue of the further layer a decor is applied to the substrate.

It is also possible to use a luster color which usually comprises dissolved metal resinates which at higher temperatures form a thin metal oxide layer with a colored or metallic appearance.

Moreover it is possible to use a metal or metal oxide containing lacquer.

The patterned sol-gel layer is preferably transparent and may for example be applied by a screen printing process. In one embodiment of the invention, the sol-gel layer is patterned using an embossing tool, in particular an embossing stamp or a roller.

An advantage of this embodiment of the method is that it permits to copy almost any texture from a master pattern, for example that of a brushed metal surface, and to transfer it to the sol-gel layer. The subsequently applied further layer follows the pattern of the patterned sol-gel layer or fills it.

Such a method comprises for example the following steps:

First, a stamp is produced for which a master is required. For a brushed stainless steel surface this may be a sheet metal or piece of metal having the desired pattern, for example. The master may be cleaned so as to be free of grease and lint.

Then, a polymer mass, in particular a silicone molding compound, is poured onto the master and cured. Curing may be accomplished for example by polymerization, preferably under heating. Possibly occurring bubbles may be removed from the system by vacuum. The cured polymer stamps are then released from the master template and are now ready for the actual embossing process. The stamps can be used multiple times, i.e. after the embossing operation they may be cleaned, for example using a cleaning agent such as water, ethanol, or isopropanol, and can so be reused.

Subsequently, the glass or glass-ceramic substrate is coated with a sol-gel layer. The sol-gel layer may comprise amorphous and/or hybrid and/or crystalline particles. By appropriately choosing the composition of the particles, the refractive index of the layer may be adapted to that of the substrate and/or to that of an overlying layer. Refractive indices that may be obtained for the patterned layer range from 1.2 to 1.8, preferably from 1.4 to 1.65. Adaptation of the refractive index is particularly important if the deposited layer serves for compensating a color cast of the substrate. An advantage of a sol-gel layer is that it may be applied by virtually any known liquid coating technique. Preferably, the sol-gel layer is applied by a screen printing process. An advantage thereof is that areas which are not intended to be provided with a decorative layer may be excluded from being coated, and that a sophisticated inspection for layer roughnesses, inhomogeneities, and/or contaminations may be dispensed with in these areas.

Depending on the kind of sol that is employed, the sol may be precured prior to embossing, in particular using a photochemical process, for example by means of UV radiation, or thermally, or using IR radiation.

Then, the sol-gel layer may be embossed using the polymer embossing stamp, in continuous or static manner. The embossing stamp may likewise be provided with a sol-gel layer, as it is envisaged according to one embodiment of the invention. Preferably under a defined pressure of the stamp, the arrangement may be cured thermally and/or photochemically, preferably photochemically. In another embodiment, a sol is employed which is automatically drawn into the stamp, due to capillary forces. In a particular embodiment, the embossing process may be a thixotropic process.

Upon removal of the polymer embossing stamp, a patterned layer is exhibited, especially having a layer thickness from 0.01 to 1 mm.

The pattern is predefined by the pattern of the embossing stamp, and nearly any pattern is possible, for example periodic or random patterns. When using a stamp almost any surface profile may be copied, for example wooden surfaces, etched surfaces, etc.

The pattern depth may vary within a range from 100 nm to 1 mm. The applied patterned layer defines the texture of the at least one subsequent further layer.

In a preferred embodiment of the invention, the patterned layer is cured, for example at temperatures from 50 to 1000° C., preferably from 100 to 500° C., depending on the required temperature resistance of the final product. The pattern is maintained, even if the organic components of the sol are removed at least partially, i.e. the layers are either almost purely inorganic or partially organic and may in particular comprise high-temperature stable Si-methyl and/or Si-phenyl groups.

However it may happen that the pattern depth decreases as compared to that of the master, depending on the temperature and the sol used, in particular depending on whether or not nanoparticles are added to the sol and in which amount. So a reduction of the pattern depth of up to 60% may result. This may be adjusted selectively to permit to produce different pattern depth with one master.

The resultant patterned sol-gel layer may have a refractive index ranging from 1.2 to 1.8, preferably from 1.4 to 1.65. The layer may be porous, i.e. it has meso- and/or micropores with a pore radius of not more than 10 nm, preferably not more than 5 nm, more preferably 3 nm. Most of the pores are bottleneck-shaped, with the pore radius being larger than the outlet radius. The layers preferably exhibit a porosity of from 5 to 50%, more preferably from 15 to 30%. The pore radii and the total porosity can be determined by ellipsometric porosimetry or by nitrogen-sorption measurements.

The porosity of the layer, through a mixture of the coating material and air, may also contribute to achieve an adaptation of the refractive index.

Sol-gel precursors that may be used are in particular hydrolyzed and condensed epoxy- or methacrylate-functionalized alkoxysilanes which in particular comprise oxide nanoparticles such as SiO₂. The nanoparticles are preferably added as a dispersion, in particular as an alcoholic dispersion. This allows to decrease shrinking to 25% or less.

In particular irregularly shaped fibrous particles may be used, especially silicon oxide particles. The diameter thereof may range from 5 to 15 nm, with a length from 5 to 150 nm.

In other embodiments, mixtures of spherical particles of different sizes are added, in particular sizes from 5 to 125 nm. Thereby, in particular when using larger and smaller particle fractions, it is possible to provoke self-assembly.

In an alternative embodiment, a self-assembling sol-gel is used. This is a self-assembling layer for which different diffuser particles are added, especially of a size ranging from 0.1 to 1 μm, which due to the different particle sizes arrange themselves, at the latest during drying and condensation of the layer, such that a pattern with a defined roughness results, which may function for example as a diffusion layer or as a layer having an anti-slip effect.

Coating using a so-called self-assembling sol also provides for a continuous cost-efficient and production-ready application process. The sol used therefor also preferably comprises amorphous and/or hybrid and/or crystalline nanoparticles. Additionally, it preferably comprises a proportion of texture-defining inorganic particles, in particular of a size ranging from 0.1 to 1 p.m. These particles may for example serve as scattering particles. Through the choice of different particle sizes, surface roughnesses and scattering properties of the layers may be adjusted. A self-assembling sol-gel layer may also be applied by a screen printing process, for example.

Use of a self-assembling sol is particularly suitable for applications, especially decorative applications, which are desired to have a matted surface.

Then, the further layer is applied using a vapor based deposition process, preferably a PVD or CVD process, more preferably a sputtering process. This layer may be applied over the entire surface, or may be patterned in a sense of sub-sections. In the latter case techniques known in the art are employed, such as peel-off films or the like.

In many cases, PVD (Physical Vapor Deposition) and CVD (Chemical Vapor Deposition) permit to transfer a target or precursor material onto the substrate in a particularly simple way. In a sputtering process, when using a metal target, the material may be oxidized by supplying gaseous O₂, where appropriate, and so an oxide layer may be applied. Such a process may also be achieved with other additives and is referred to as so-called reactive sputtering.

Besides oxides, nitrides or elementary layers may be used as well. In particular, the following elements or compounds may be used: Si, Ge, Al, Mn, Fe, Zn, Ni, Cr, Ti, Mo, Ag, NiCr, NiCrSi (so-called Thermax® stainless steel), and/or alloys or combinations thereof.

In another embodiment, metal clusters having a metallic appearance are introduced or embedded in the sputtered layer, in nanoparticulate form.

Problematic, in particular in this embodiment of the invention, is a possible color deviation of the resulting composite material due to the so-called Yellowness index of the glass-ceramic and the thermal resistance, in particular of metallic layers, since temperature loads of more than 100° C. may lead to oxidation.

The inventors have found that at least one further layer applied by a deposition process may serve as a color adjusting layer, i.e. for example may compensate for the yellowness of the glass-ceramic substrate.

By selecting an appropriate color adjusting layer, especially when selecting a layer having a dispersion of n>1.55, along with the colored transmittance of the glass-ceramic, a uniform transmittance without color appearance may be achieved, or a specific other color cast, such as a blue tint. A colorless or color-adjusted appearance is to be understood in particular as a transmission, wherein when transmitting white light, the transmitted, i.e. the passed light has an absolute value below 20, preferably below 10, on the a* axis and/or the b* axis in the L*a*b* color space. Thus, a preferred embodiment of the composite material does not exhibit any color cast, not greenish, nor reddish, or bluish, or yellowish.

This may for example be achieved by an Si₃N₄ layer or an SiO_(X)N_(V) layer with a thickness ranging for example from 40 to 500 nm, in particular ranging from 75 to 97 nm.

Besides single layers, two-layer or multilayer systems of the following materials may be used to reduce the intrinsic color of glass-ceramics: Si_(X)N_(V), SiO_(X)N_(V), TiO₂, ZrO₂, Y₂O₃, MgF₂, Al₂O₃, Nb₂O₅, Ta₂O₅, CeO₂, and/or SiO₂.

A combination of high refractive index layers and low refractive index layers such as titanium oxide and silicon oxide is likewise conceivable.

In another embodiment of the invention, the colored layer, in particular if applied on a metallic-looking layer, in particular a metal or metal oxide layer, may serve as an additional gas barrier and so ensure thermal resistance up to 800° C.

In order to ensure that under thermal stress the layer for compensating a color cast of the substrate is not or only slightly altered, optionally at least one further layer, preferably a barrier layer, is sputtered with high ion energies, and the substrate is heated prior to sputtering. Typical ion energies here are greater than 10 eV, preferably greater than 20 eV, and the substrate heating temperatures range from 100 to 600° C., preferably from 200 to 400° C.

Since many metallic coatings may degrade at the atmosphere under high thermal stress, a further layer as a barrier layer is useful on the coloring or color-compensating and/or on the metallic layer. Here, this layer may be formed as a hard material coating.

It may for example comprise silicon, aluminum, titanium, tin, zirconium, cerium or oxides or nitrides or oxynitrides thereof. In particular silicon oxide, silicon oxynitride, and silicon nitride are envisaged for barrier layers. For ensuring durable protection, barrier layers with thicknesses ranging from 50 to 500, preferably from 100 to 250 nm are suitable.

In another embodiment of the invention, a further layer is provided between the glass-ceramic substrate and the sol-gel layer, this layer in particular also being applied by a PVD process, especially by a sputtering process. This layer preferably exhibits a CTE of less than 5*10⁻⁶ K⁻¹.

The layer serves as an adhesion promoting layer for the sol-gel layer and any further layers. This layer may also be colored and thus serve as a color adjusting layer, as envisaged according to one embodiment of the invention.

In a refinement of the invention, in order to provide for a touch-enabled layer, in particular when using capacitive and/or inductive switches behind control panels, a thin layer having a high resistivity is applied. Since pure metal layers typically would need to be made so thin that transparency would be too high for many applications, semiconductor materials such as silicon or germanium may be used for this purpose. In particular a semiconductor material is used which at room temperature exhibits a resistivity of more than 1*10³ Ωmm²/m, preferably of 1*10⁶ Ωmm²/m.

In this way, a touch-enabled opaque layer with an appearance of stainless steel, in particular brushed stainless steel may be provided, for example. Semiconductor materials in doped form are likewise conceivable, for example those including aluminum, boron, or phosphorus.

In order to improve the resistance of the layer system to oil, water, glue, acids, or sugar compounds, as is provided for in one refinement of the invention, a sealing coating may be applied, especially with one or more preferably temperature-stable sol-gel layers and/or glass flux and/or phenyl or methyl polysiloxane layers. Furthermore, the layer may be pigmented by inorganic fillers, in particular pyrogenic or precipitated inorganic materials, and/or by mica and/or graphite and/or inorganic pigments, in particular black pigments. Surprisingly, a layer system with such a sealing coating may comply with a scratch resistance greater than 300 g, preferably greater than 500 g in the Bosch-Siemens home appliance test (BSH) known in the art.

Moreover, the at least partially inorganic layer or the deposited layer may serve to enhance the surface hardness of the composite material.

In one embodiment of the invention, the composite material is at least partially transparent, so that for example infrared sensors may be arranged behind the composite material. Preferably there is no sealing coating in this area.

It has been found that the layer systems according to the invention do not alter the strength of a glass-ceramic article.

Since PVD and CVD methods permit a deposition of layers even at a relatively low substrate temperature, the invention is also suitable for transparent plastics, such as polycarbonates, polyacrylates, polyolefins, in particular cyclo-olefinic copolymers. Besides the use in the field of white goods (e.g. kitchen appliances), the composite material according to the invention may also be used indoors and for architectural glazing, and applications in consumer electronics are also conceivable.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to FIGS. 1 to 5, a method for producing a composite material with a metallic appearance will be described in more detail with reference to a schematically illustrated exemplary embodiment.

FIG. 6 schematically shows a flow chart of one exemplary embodiment of a method for producing a composite material having an appearance of brushed stainless steel.

FIGS. 7 and 8 show transmission curves of a composite material with and without a colored layer to compensate for the color cast of the substrate.

FIGS. 9 and 10 show exemplary layer systems for a glass-ceramic coating with an appearance of stainless steel (FIG. 9) and for a touch-enabled glass-ceramic coating with an appearance of stainless steel (FIG. 10).

Referring to FIG. 11, the color appearance of several layer systems following thermal stress will be explained.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a composite material 1 which comprises a substrate 2 on which a sol-gel layer 3 is applied as the layer to be patterned in sections thereof.

As illustrated in FIG. 2, the sol-gel layer 3 is patterned using an embossing tool 4 that is pressed onto the layer.

The patterned sol-gel layer 3 is illustrated in FIG. 3. Preferably, the embossed pattern is similar to the texture of a brushed stainless steel surface.

Then, as illustrated in FIG. 4, a metal layer 5 is applied onto patterned sol-gel layer 3 using a sputtering process, and the result is a coating having a metallic appearance.

In order to correct a color cast of the substrate another layer may be applied between the patterned layer and the metal layer, which leads to a substantially colorless transmittance and/or the desired color adjustment.

As an embossing tool an embossing stamp may be used, for example.

The sol-gel layer may be applied onto a glass or glass-ceramic substrate by liquid coating, for example. Preferably the sol-gel layer comprises amorphous and/or hybrid and/or crystalline particles. Through the particles and the sol, an adaptation of the refractive index of the layer to that of the substrate 2 or to that of further overlying layers (not shown) may be provided.

Preferably, the sol-gel layer 3 is applied using a screen printing process. Therefore, areas which are not to be patterned may be excluded from being coated, and free areas which are for example required for a display or a control panel are not provided with the sol-gel layer, and a sophisticated inspection for layer roughnesses, inhomogeneities, and/or contaminations in these areas is eliminated.

In one embodiment of the invention, sol-gel layer 3 is precured, in particular using a photochemical process.

The embossing tool 4 is preferably formed as an embossing stamp, which is continuously pressed onto the sol-gel layer under a defined pressure, in particular in form of a roller or a planar embossing stamp. In one embodiment of the invention, the sol-gel layer 3 is cured during embossing, in particular thermally or photochemically. Especially when a relatively low-viscosity sol is used, there is no need to apply the embossing stamp with particularly great force, rather the sol is drawn into the texture of the embossing stamp almost automatically. If bubbles arise between the embossing stamp and the sol, they may be removed using a roller, or assisted by vacuum.

As a pattern, which is in particular understood as a three-dimensional surface profile, both periodic and random patterns may be used, in particular lines, brushed surfaces, light-scattering layers, etched surfaces, textures that serve to provide enhanced haptics, and simple geometric shapes such as pyramids, inverted pyramids, cubes, spheres, etc.

In one particular embodiment, the patterned sol-gel layer defines the texture of subsequently deposited layers, in particular that of the metal-containing layers deposited according to the invention, and optionally also that of subsequent further layers or intermediate layers.

The patterned sol-gel layer 3 provided with a metal layer 5 is cured at temperatures from 50 to 1000° C., preferably from 100 to 800° C., depending, inter alia, on the required temperature resistance of the final product.

The pattern is maintained, even if organic components of the sol-gel layer are completely burned off.

The depth of the pattern decreases by 0 to 60% as compared to the pattern of the embossing tool, depending on the temperature and the sol-gel used which in particular includes particles. If necessary, this may be compensated for by using an embossing tool that has a greater pattern depth than the pattern depth that is actually desired.

FIG. 6 shows a schematic flow chart of the essential method steps.

First, a sol-gel layer is applied. The applied sol-gel layer is patterned using an embossing stamp and is simultaneously cured using UV light. Then, the embossing stamp is removed and subsequently the composite material is cured thermally.

Finally, a stainless steel layer is applied by a sputtering process. If desired, additional layers may be applied.

In detail, a composite material according to the invention may be produced as follows, for example:

Example 1

In a vessel, 22.3 g (0.08 mol) of GPTES (glycidyloxypropyltriethoxysilane) is provided with 4.1 g (0.02 mol) of TEOS (tetraethoxysilane), and is hydrolyzed with 2.3 g of water in which 0.344 g of PTSA (p-toluenesulfonic acid) had been dissolved.

After stirring for 2 min, 110 g of a 15 mass-% alcoholic dispersion of irregularly shaped SiO₂ nanoparticles in isopropanol is added to this hydrolyzate. The nanoparticles have a fibrous shape for example, with a diameter from 5 nm to 15 nm and a length from 30 to 150 nm. Into this solution 13 g of ethylene glycol monoethyl ether is added, and the highly volatile solvent is removed in a rotary evaporator at 100 mbar and 50° C. bath temperature. Then, 0.6 g of the cationic photoinitiator Irgacure® 250 in 1 g of ethylene glycol monoethyl ether is added to the embossing sol. By screen printing using a 187 mesh, layers of this lacquer system may be applied to one face of a soda lime glass.

After the solvent has been dried off between room temperature and 50° C., with or without air circulation, a patterned silicone stamp (PDMS) is applied, and then the layer is cured through the stamp using a UV lamp. When the stamp is removed the pattern of the embossing stamp had been transferred into the nanoparticle-functionalized coating.

Optionally, the layers may then be cured thermally at temperatures from 300 to 500° C. A preferred heating rate is 3 K/min, with a holding time of 1 h at 500° C. The final thickness of the layer ranges from 2 to 3 p.m.

Subsequently, a stainless steel layer is applied by a sputtering process in a thickness of at least 100 nm.

In this way, a temperature-stable composite material with an appearance of brushed stainless steel is provided.

Example 2

In a vessel, 16.7 g (0.06 mol) of GPTES (glycidyloxypropyltriethoxysilane) is provided with 4.1 g (0.02 mol) of TEOS (tetraethoxysilane) and 3.56 g (0.02 mol) of MTEOS (methyltriethoxysilane), and is hydrolyzed with 2.3 g of water in which 0.344 g of PTSA (p-toluenesulfonic acid) had been dissolved.

After having been stirred for 2 min, a mixture of 44 g of a 30 mass-% alcoholic dispersion of spherically shaped SiO₂ nanoparticles with a diameter from 40 to 50 nm in isopropanol and 11 g of an alcoholic dispersion of spherical SiO₂ nanoparticles with a diameter from 10 to 15 nm is added to this hydrolyzate. 16 g of ethylene glycol monoethyl ether is added to this solution, and the highly volatile solvent is removed in a rotary evaporator at 100 mbar and 50° C. bath temperature. Then, 0.6 g of the cationic photoinitiator Irgacure® 250 in 1 g of ethylene glycol monoethyl ether is added to the embossing sol.

By screen printing using a 187 mesh, layers of this lacquer system may be applied to one face of transparent glass-ceramic substrates.

After the solvent has been dried off between room temperature and 50° C., with or without air circulation, a patterned silicone stamp (PDMS) is applied, and then curing is accomplished through the stamp using a UV lamp. When the stamp is removed the pattern of the stamp had been transferred into the nanoparticle-functionalized lacquer.

Optionally, the layers may then be cured thermally at temperatures from 300 to 500° C. A preferred heating rate is 3 K/min, with a holding time of 1 h at 500° C. The final thickness of the layer is 1.5 p.m.

Then, using the sputtering process, first a color-adjusting SiN layer is applied in a thickness of 80 nm, then a stainless steel layer in a minimum thickness of 100 nm, and then an SiO₂ barrier layer as the top layer.

In this manner a composite material is provided which has no color cast in transmission, i.e. in which the yellowness of the substrate is compensated for, and which exhibits high scratch resistance. In addition, the metallic layer is protected from oxidation.

FIG. 7 shows the transmission curve of a known non-coated glass-ceramic substrate. The x-axis 8 indicates the wavelength in nm, and the y-axis 9 indicates the transmittance 7 in %.

It can be seen that the transmittance continuously increases from slightly above 91.5% at 400 nm to 92.5% at 650 nm. Accordingly, the glass-ceramic material is yellowish.

FIG. 8, similar to FIG. 7, shows the transmission curve of a glass-ceramic substrate coated with a color adjusting layer.

It can be seen that the transmission curve 7 in the wavelength range from 400 to 750 nm substantially extends at about 93%, that is, the composite material has no visible color appearance.

In this exemplary embodiment, color adjustment has been achieved by bringing the transmittance in the entire visible range to substantially the same level.

It will be appreciated that, instead, it is likewise conceivable to effect a desired, non-constant transmission curve which by virtue of individual peaks of the color adjusting layer has a spectral distribution which is substantially colorless to the human eye, in particular one that takes an absolute value below 10 on both the a* axis and the b* axis in the L*a*b* color space.

FIG. 9 shows an exemplary layer structure of a composite material having a metallic appearance, especially an appearance of brushed stainless steel.

On a glass or glass-ceramic substrate, first a patterned sol-gel layer is applied, then followed by a layer for color compensation. On this layer a metal layer is disposed.

This embodiment of the invention is particularly suitable for coating a bottom face or for use of the layer system on the side facing away from the viewer.

The patterned sol-gel layer functions to define a pattern of the subsequent layers. In this embodiment, the layer for color compensation of a yellowish tint of the glass or glass-ceramic substrate is arranged between the metal layer and the patterned layer.

It will be appreciated, however, that another embodiment in which the layer for color compensation is arranged between the glass-ceramic substrate and the patterned sol-gel layer is likewise conceivable.

FIG. 10 shows an alternative layer structure which is in particular intended for a touch-enabled bottom face coating with an appearance of stainless steel. Also in this embodiment, first a patterned sol-gel layer is applied on the glass-ceramic substrate. It is followed by a silicon nitride layer which serves for color compensation.

To create a metallic appearance, a semiconductor layer follows, in this embodiment a silicon layer, which due to its high electrical resistance also permits to attach capacitive switching elements behind the composite material.

Then another silicon nitride layer follows, which ensures a sufficient temperature resistance of the composite material, for example for glass-ceramic cooktops or oven viewing windows.

The table of FIG. 11 shows the color impression of several composite materials before and after temperature stress.

Listed in the table are layer systems applied to a glass-ceramic substrate and having a metallic appearance, in which first a silicon nitride layer was applied in different thicknesses as a color adjusting layer, onto which then a stainless steel layer in a thickness of 300 nm and a silicon oxide layer in a thickness of 300 nm as a barrier layer were applied.

The coordinates of the L*a*b* color space are indicated in the unstressed state and following a thermal stress of 490° C. It can be seen that with increasing thickness of the silicon nitride layer the brightness (L* axis) tends to increase slightly. The absolute values on the a* axis and on the b* axis are always less than 8, both for the unstressed samples and for the samples stressed at a temperature of 490° C.

Furthermore, it can be seen that the maximum indicated level of a color change dE is about 2.5, i.e. the color appearance of the composite material has not or only hardly visibly changed, even after exposure to heat.

The invention permits to produce temperature-resistant composite materials with virtual any visual appearance in a very simple way. 

1-24. (canceled)
 25. A method for producing a glass or glass ceramic composite material, comprising the steps of: providing a glass or glass ceramic substrate with a coefficient of thermal expansion of α≦5*10⁻⁶ K⁻¹; applying a sol-gel layer onto the substrate; adding particles to the sol-gel layer; patterning the sol-gel layer; thermally curing the sol-gel layer; and applying at least one further layer using a deposition process.
 26. The method as claimed in claim 25, wherein the further layer is deposited by a PVD or CVD process.
 27. The method as claimed in claim 25, wherein the further layer is a colored layer or a layer having a metallic appearance.
 28. The method as claimed in claim 25, wherein the further layer is a layer that includes at least one semiconductor material.
 29. The method as claimed in claim 25, wherein the theramally curing step is performed before the patterning step.
 30. The method as claimed in claim 25, further comprising adding nanoparticles to the sol-gel layer, the nanoparticles having a mean particle size from 0.1 to 1 μm.
 31. The method as claimed in claim 25, wherein the thermally curing step comprises thermally curing at 50 to 1000° C.
 32. The method as claimed in claim 25, wherein the thermally curing step comprises thermally curing at 100 to 500° C.
 33. The method as claimed in claim 25, wherein the patterning step comprises using an embossing stamp or a roller on the sol-gel layer.
 34. The method as claimed claim 25, wherein the adding step comprises adding particles to a precursor of the sol-gel layer in a volume fraction that ranges from 0.1 to 0.9.
 35. The method as claimed claim 34, wherein the volume fraction ranges from 0.5 to 0.8.
 36. The method as claimed in claim 25, wherein the sol-gel layer comprises a self-assembling sol-gel.
 37. A method for producing a composite material having at least two layers, comprising the steps of: applying a patterned sol-gel layer onto a substrate; and applying at least one further layer using a deposition process.
 38. A composite material, comprising: a glass or glass ceramic substrate, the substrate having a coefficient of thermal expansion of α≦5*10⁻⁶ K⁻¹; an at least partially inorganic layer disposed on the substrate, the at least partially inorganic layer having a pattern defined thereon; and a further layer disposed on the at least partially inorganic layer.
 39. The composite material as claimed in claim 38, wherein the further layer is a PVD or CVD deposited layer.
 40. The composite material as claimed in claim 39, wherein the at least partially inorganic layer is porous.
 41. The composite material as claimed in claim 38, wherein the at least partially inorganic layer and/or the further layer has/have a strength that is at least equal to a strength of the substrate.
 42. The composite material as claimed in claim 38, wherein at least partially inorganic layer has a refractive index that ranges from 1.2 to 1.8 and/or the further layer has a refractive index that ranges from 1.2 to 2.2.
 43. The composite material as claimed in claim 42, wherein the refractive index of the at least partially inorganic layer ranges from 1.4 to 1.65 and the refractive index of the further layer ranges from 1.8 to 2.1.
 44. The composite material as claimed in claim 38, wherein the at least partially inorganic layer comprises high-temperature stable Si-methyl and/or Si-phenyl groups.
 45. The composite material as claimed in claim 44, further comprising a transmission curve in a wavelength range from 400 to 750 that has a minimum that differs from a maximum by less than 0.6 percentage points.
 46. The composite material as claimed in claim 38, wherein the substrate exhibits a colored transmittance which is compensated for by a colored transmittance of the further layer such that the composite material has a substantially colorless transmittance.
 47. The composite material as claimed in claim 38, wherein, when transmitting white light therethrough, the transmitted light has an absolute value below 20 on the a* axis and/or the b* axis in the L*a*b* color space.
 48. The composite material as claimed in claim 47, wherein the absolute value is below
 10. 49. The composite material as claimed in claim 47, wherein the at least partially inorganic layer has a pattern depth that ranges from 0.05 μm to 2 mm.
 50. The composite material as claimed in claim 38, wherein the at least partially inorganic layer has a thickness from 0.01 to 1 mm.
 51. The composite material as claimed in claim 38, wherein the further layer has a thickness from 10 nm to 100 p.m.
 52. The composite material as claimed in claim 38, wherein the further layer has a thickness from 80 nm to 30 p.m.
 53. The composite material as claimed in claim 38, wherein the further layer comprises a semiconductor material and exhibits a resistivity at room temperature of more than 1*10³ Ωmm²/m.
 54. The composite material as claimed in claim 38, wherein the composite material forms part of a kitchen appliance. 